Honeycomb structure

ABSTRACT

A honeycomb structure includes a honeycomb segment joined body comprising a plurality of honeycomb segments unitarily joined at joint faces thereof by means of a bonding material layer, and an outer peripheral coat layer, the honeycomb structure having a plurality of cells functioning as fluid passages disposed in parallel with one another in a central axis direction. A thickness of the outer peripheral coat layer at least at one portion of the portions (portions on end faces) located on end faces in longitudinal direction of the bonding material layer in a cross-section in a direction perpendicular to the central axis direction is larger than an average thickness of the portions other than the portions on end faces.

BACKGROUND

This is a Continuation of application Ser. No. 11/474,323 filed Jun. 26,2006. This application claims the benefit of Japanese Patent ApplicationNo. 2005-199209, filed Jul. 7, 2005. The entire disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

The present invention relates to a honeycomb structure having aplurality of honeycomb segments unitarily joined by means of a bondingmaterial layer. More specifically, the present invention relates to ahoneycomb structure useful as a trapping filter for exhaust gas,particularly as a diesel particulate filter (DPF) for trappingparticulate matter or the like contained in exhaust gas from a dieselengine, and capable of effectively inhibiting generation of defects suchas cracks due to thermal stress by lightening an influence of thermalstress generated upon production.

A honeycomb structure is incorporated in an exhaust system or the likeof a diesel engine as a trapping filter for exhaust gas, for example, asa diesel particulate filter (DPF) in order to trap and removeparticulate matter contained in exhaust gas from a diesel engine or thelike. Such a honeycomb structure has a problem of generating defectssuch as a crack due to thermal stress generated on an outer peripheralcoat layer (particularly, in a portion located on an end face in thelongitudinal direction of the bonding material layer in a cross-sectionin a direction perpendicular to a central axis direction). Though it canbe considered to lower a cooling rate in order to reduce such defects,it is not the best measure since it lowers productivity.

Particularly, a honeycomb structure of silicon carbide is expectedbecause it is excellent in thermal resistance. However, it has a highthermal expansion coefficient (high thermal stress generated) and lowthermal shock resistance in comparison with a cordierite honeycombstructure, and the aforementioned problem is amplified. Further,remarkable increase in thermal stress caused in accordance with recentincrease in size of a filter causes more serious generation of defectssuch as a crack more frequently to higher extent.

SUMMARY

The present invention has been made in view of the above problem andaims to provide a honeycomb structure useful as a trapping filter forexhaust gas, particularly as a diesel particulate filter (DPF) fortrapping particulate matter or the like contained in exhaust gas from adiesel engine, and capable of effectively inhibiting generation ofdefects such as cracks due to thermal stress by lightening an influenceof thermal stress generated upon production.

In order to achieve the above aim, there is provided the followinghoneycomb structure according to the present invention.

[1] A honeycomb structure comprising:

-   -   a honeycomb segment joined body comprising a plurality of        honeycomb segments unitarily joined at joint faces thereof by        means of a bonding material layer, and    -   an outer peripheral coat layer covering an outer peripheral        surface of said honeycomb segment joined body,    -   the honeycomb structure having a plurality of cells functioning        as fluid passages disposed in parallel with one another in a        central axis direction;    -   wherein a thickness of said outer peripheral coat layer at least        at one portion of the portions (portions on end faces) located        on end faces in longitudinal direction of the bonding material        layer in a cross-section in a direction perpendicular to the        central axis direction is larger than an average thickness of        the portions other than the portions on end faces.

[2] A honeycomb structure according to the above [1], wherein a maximumthickness (t₁) in a portion (maximum portion) having the maximumthickness in the portions on end faces and the average thickness (t₂) inthe portions other then the portions on end faces of said outerperipheral coat layer satisfy the relation of (t₁)≧2×(t₂).

[3] A honeycomb structure according to the above [1], wherein a maximumthickness (t₁) in a portion (maximum portion) having the maximumthickness in the portions on end faces and the average thickness (t₂) inthe portions other then the portions on end faces of said outerperipheral coat layer satisfy the relation of (t₁)≧(2 to 40)×(t₂).

[4] A honeycomb structure according to the above [1], wherein athickness (t₃) of said outer peripheral coat layer at the four portions(portions on end faces in 90° directions) located in 90° direction withone another on end faces in longitudinal direction of the bondingmaterial layer in a cross-section in a direction perpendicular to thecentral axis direction and the average thickness (t₄) in the portionsother then the portions on end faces in 90° directions of said outerperipheral coat layer satisfy the relation of (t₃)≧(2 to 40)×(t₄).

[5] A honeycomb structure according to any one of the above [1] to [4],wherein a cross-section, in a direction perpendicular to the centralaxis direction, of the honeycomb segments has a rectangular shape.

[6] A honeycomb structure according to any one of the above [1] to [5],wherein a cross-section, in a direction perpendicular to the centralaxis direction, of the honeycomb segment joined body has a shape of acircle, an ellipse, or a racetrack.

[7] A honeycomb structure according to any one of the above [1] to [6],wherein the honeycomb segments constituting the outer most periphery ofthe honeycomb structure do not have a crust having a pitch of 7 cells ormore.

As described above, according to the present invention, there isprovided a honeycomb structure useful as a trapping filter for exhaustgas, particularly as a diesel particulate filter (DPF) for trappingparticulate matter or the like contained in exhaust gas from a dieselengine, and capable of effectively inhibiting generation of defects suchas cracks due to thermal stress by lightening an influence of thermalstress generated upon production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing an embodiment (Awhole cross-section along a plane perpendicular to the central axis hasa circular shape.) of a honeycomb structure of the present invention.

FIG. 2 is a front view, from an end face, of a part of anotherembodiment (A whole cross-section along a plane perpendicular to thecentral axis has a square shape.) of a honeycomb structure of thepresent invention.

FIG. 3 is a perspective view schematically showing a honeycomb segmentused in another embodiment of a honeycomb structure of the presentinvention.

FIG. 4 is a cross-sectional view along the A-A line in FIG. 3.

FIG. 5 is a cross-sectional view schematically showing the difference inthickness depending on portions of the outer peripheral coat layer in anembodiment (A cross-section of the honeycomb segment joined body has analmost square shape.) of a honeycomb structure of the present invention.

FIG. 6 is a cross-sectional view schematically showing the difference inthickness depending on portions of the outer peripheral coat layer in anembodiment (A cross-section of the honeycomb segment joined body has analmost rectangular shape.) of a honeycomb structure of the presentinvention.

FIG. 7 is a cross-sectional view schematically showing the definition ofthickness of the outer periphery coat layer in an embodiment of ahoneycomb structure of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As shown in FIGS. 1 to 4, a honeycomb structure 1 is a honeycombstructure 1 including a honeycomb segment joined 10 comprising aplurality of honeycomb segments 2 unitarily joined at joint facesthereof by means of a bonding material layer 9, and an outer peripheralcoat layer 4 covering an outer peripheral surface of said honeycombsegment joined body 10. As shown in FIGS. 5 and 6 (FIG. 5 shows a casethat a cross-section of the honeycomb segment joined body 10 has analmost square shape, and FIG. 6 shows a case that a cross-section of thehoneycomb segment joined body 10 has an almost rectangular shape), it ischaracterized in that a thickness of the outer peripheral coat layer 4at least at one portion of the portions (portions on end faces) 4Alocated on end faces in longitudinal direction of the bonding materiallayer 9 in a cross-section in a direction perpendicular to the centralaxis direction is larger than an average thickness of the portions otherthan the portions 4A on end faces. To be concrete, a maximum thickness(t₁) in a portion (maximum portion) having the maximum thickness in theportions 4A on end faces and the average thickness (t₂) in the portionsother then the portions on end faces of the outer peripheral coat layer4 preferably satisfy the relation of (t₁)≧2×(t₂). In addition, a maximumthickness (t₁) in a portion (maximum portion) having the maximumthickness in the portions 4A on end faces and the average thickness (t₂)in the portions other then the portions 4A on end faces of the outerperipheral coat layer 4 further preferably satisfy the relation of(t₁)≧(2 to 40)×(t₂), and particularly preferably satisfy the relation of(t₁)≧(15 to 25)×(t₂). Here, thickness of the outer peripheral coat layer4 means a “distance T from the tip portion S of a partition wall incontact with the outer peripheral coat layer 4 to a surface of the outerperipheral coat layer 4 as shown in FIG. 7 in the case that thehoneycomb segments 2 do not have a crust. In the case that the honeycombsegments 2 have a crust, it means a “distance from a surface of a crust(not illustrated) of a honeycomb segment in contact with the outerperipheral coat layer 4 to a surface of outer peripheral coat layerthough it is not illustrated. In addition, the average thickness (t₂)means the “average value of the values measured at 12 points at regularintervals in portions other than the portions 4A on an end face havingthe maximum thickness (t₁) of the outer peripheral coat layer.

In the present invention, when (t₁) is below twice (t₂), many defectssuch as cracks are sometimes generated in the outer peripheral coatlayer. When (t₁) is above 40 times (t₂), productivity is sometimeslowered remarkably, or the outer peripheral coat layer formed sometimespeels off partially.

In the present invention, as shown in FIGS. 5 and 6, a thickness (t₃) ofsaid outer peripheral coat layer at the four portions (portions on endfaces in 90° directions) located in 90° direction with one another onend faces in longitudinal direction of the bonding material layer in across-section in a direction perpendicular to the central axis directionand the average thickness (t₄) in the portions other then the portionson end faces in 90° directions of the outer peripheral coat layerpreferably satisfy the relation of (t₃)≧(2 to 40)×(t₄), more preferablysatisfy the relation of (t₃)≧(15 to 25)×(t₄). Here, the averagethickness (t₃) means the average value of thickness of the outerperipheral coat layer on the end face portions 4B located in 90°directions with one another in a cross-section in a directionperpendicular to a central axis direction, and the average thickness(t₄) means the average value of values measured at 12 points at regularintervals in portions other than the portions 4B on an end face. When(t₃) is below twice (t₄), many defects such as cracks are sometimesgenerated in the outer peripheral coat layer. When (t₃) is above 40times (t₄), productivity is sometimes lowered remarkably, or the outerperipheral coat layer 4 formed sometimes peels off partially. In thepresent invention, the outer peripheral coat layer 4 is constituted sothat a layer thickness in portions 4A on end faces and portions 4B onend faces located in 90° directions is larger than a thickness in theother portions. This is because it has high probability of causingdefects such as cracks by the influence of thermal stress in theseportions, and it is to improve thermal shock resistance in theseportions.

In the present invention, it is preferable that wherein the honeycombsegments 2 constituting the outermost periphery of the honeycombstructure 1 do not have a crust having a pitch of 7 cells or more. It ismore preferable that the crust on the outermost peripheral surface has apitch of 3 cells or less, and it is particularly preferable that thehoneycomb segments 2 do not have any crust on the outermost peripheralsurface at all. When the honeycomb segments 2 constituting the outermostperiphery of the honeycomb structure 1 have a crust on the outermostperipheral surface thereof, it inhibits the cell structure from firmlybonding with the outer peripheral coat layer 4 directly. Therefore,bonding strength in this portion is low. When a thickness of the cruston the outermost peripheral surface is above 7 cell pitch, an effect ofthickening the outer peripheral coat layer 4 adjacent to this portion isnot sufficiently exhibited occasionally.

A structure of a honeycomb structure 1 in an embodiment of the presentinvention will be described more concretely. A honeycomb structure 1 inan embodiment of the present invention is constituted by a honeycombsegment joined body 10 having a plurality of honeycomb segments 2 eachhaving a structure having a plurality of cells 5 functioning as fluidpassages separated and formed by porous partition walls 6 disposed inparallel with one another in a central axis direction, the honeycombsegments 2 being unitarily joined by means of a bonding material layer9, and an outer peripheral coat layer 4 covering an outer peripheralsurface of the honeycomb segment joined body 10. After the honeycombsegments 2 are joined by means of a bonding material layer 9, a grindingprocess is conducted to give the whole cross-section along a planeperpendicular to the central axis of the honeycomb structure 1 a shapeof a circle, an ellipse, a racetrack, or a partially transformed shapethereof. Then, the outer peripheral surface is covered with an outerperipheral coat layer 4. When the honeycomb structure 1 is used as aDPF, particulate matter containing soot discharged from a diesel enginecan be trapped by arranging the honeycomb structure in an exhaust gassystem, or the like, of the diesel engine. In FIG. 1, cells 5 andpartition walls 6 are shown only in one honeycomb segment 2. As shown inFIGS. 3 and 4, each of the honeycomb segments 2 has a shape forconstituting a part of the whole structure of the honeycomb structure 1(honeycomb segment joined body 10) (see FIG. 1), and the honeycombsegments 2 are combined with one another in a direction perpendicular tothe central axis of the honeycomb structure 1 (see FIG. 1) to constitutethe whole structure. The cells 5 are disposed in parallel with oneanother in the central axis direction of the honeycomb structure 1, andeach of the end portions of the adjacent cells 5 are alternately pluggedwith a filler 7.

In the FIGS. 3 and 4, each of predetermined cells 5 (inflow cells) hasan opened end portion on its left, while each of the other cells 5(outflow cells) adjacent to inflow cells has a closed end portionplugged with the filler 7 on its left and an opened end portion on itsright. Such plugging gives each of the end faces of the honeycombsegments 2 a checkerwise pattern as shown in FIG. 2. When a honeycombstructure 1 having a plurality of such honeycomb segments 2 is arrangedin an exhaust gas system for exhaust gas, exhaust gas flows into each ofthe honeycomb segments 2 from the left side in FIG. 4 and moves towardthe right side.

FIG. 4 shows the case of the exhaust gas inlet port located on the leftof the honeycomb segment 2, where exhaust gas flows into the honeycombsegment 2 from opened cells 5 (inflow cells) without being plugged. Theexhaust gas flowed into the cells 5 (inflow cells) passes through porouspartition walls 6 and flow out from the other cells 5 (outflow cells).When the exhaust gas passes through the partition walls 6, particulatematter containing soot in the exhaust gas is trapped in the partitionwalls 6. Thus, exhaust gas can be purified. Since a pressure lossincreases because particulate matter containing soot accumulates in thecourse of time inside the honeycomb segments 2 due to such trapping,regeneration is conducted by combusting soot or the like. Though FIGS. 2to 4 show honeycomb segments 2 each having a square whole cross-section,the shape may be a square, partially transformed square, triangle,hexagon, or the like. A cross-sectional shape of a cell 5 may be apolygon, circle, ellipse, racetrack shape, or partially transformedshape thereof.

A material for the honeycomb segments 2 is preferably at least one kindselected from the group consisting of silicon carbide, silicon-siliconcarbide based composite material, silicon nitride, cordierite, mullite,alumina, spinel, silicon carbide-cordierite based composite material,silicon-silicon carbide composite material, lithium aluminum silicate,aluminum titanate, and Fe—Cr—Al based metal from the viewpoint ofstrength and thermal resistance. Of these, silicon carbide orsilicon-silicon carbide based composite material is preferable.

The honeycomb segment 2 can be produced, for example, by adding to oneselected from the above materials a binder such as methyl cellulose,hydroxypropoxyl cellulose, hydroxydiethyl cellulose, carboxymethylcellulose, and polyvinylalcohol, a surfactant, water as a solvent, andthe like, to prepare clay having plasticity; subjecting the clay toextrusion forming to give the aforementioned shape; drying the extrudedbody with microwaves, hot air, or the like; followed by sintering. Inthe present invention, the honeycomb segment 2 preferably has a thermalexpansion coefficient of 1×10⁻⁶/° C. or more. When it is below 1×10⁻⁶/°C., a large amount of thermal stress is generated with thermal shockresistance being deteriorated, which sometimes increases occurrence of adefect such as a crack in the outer peripheral wall.

As a filler 7 used for plugging the cells 5, a material similar to thatof the honeycomb segment 2 can be used. Plugging with the filler 7 canbe conducted by filling slurried filler 7 into the opened cells 5 byimmersing an end face of the honeycomb segment with the cells 5 notrequiring plugging being masked. Though the filling of the filler 7 maybe conducted either before firing or after firing after forming of thehoneycomb segment 2, it is preferably conducted before firing because itrequires only one firing step.

After production of the honeycomb segment 2 as described above, aslurried bonding material layer 9 is applied on the outer peripheralsurface of the honeycomb segment 2. A plurality of honeycomb segments 2are combined to give a predetermined solid shape (whole structure of thehoneycomb structure 1) and subjected to press fitting, followed bydrying by heating. Thus, a joined body having a plurality of honeycombsegments 2 unitarily joined with one another can be manufactured. Then,the joined body is subjected to grinding processing to give theaforementioned shape, and the outer peripheral surface is covered withan outer peripheral coat layer 4, followed by drying by heating. Thus, ahoneycomb structure 1 shown in FIG. 1 is manufactured.

The bonding material layer 9 used in the present invention is applied onthe outer peripheral surface of the honeycomb segments 2 so as to bondthe honeycomb segments 2 with one another. Though the bonding materiallayer 9 may be applied on the outer peripheral surface of each of thehoneycomb segments 2, it may be applied on only one of the correspondingouter peripheral surfaces between adjacent honeycomb segments 2.

Such application on only one of the corresponding surfaces is preferablein that an amount of the bonding material layer 9 can be saved.Thickness of the bonding material layer 9 is determined in considerationof bonding force between honeycomb segments 2 and suitably selectedwithin the range from 0.2 to 4.0 mm, for example.

The bonding material layer 9 used in the present invention preferablycontains ceramic as the main component and a granular filler. Thebonding material layer 9 and the outer peripheral coat layer 4 can usethe same material. Examples of the ceramic contained as the maincomponent in the bonding material layer 9 and the outer peripheral coatlayer 4 in the present embodiment include silicon carbide, siliconnitride, cordierite, alumina, and mullite. The material may contain acolloidal sol such as colloidal silica and colloidal alumina and, asnecessary, metal fibers and a pore former besides the ceramic as themain component.

The outer periphery coat layer 4 is applied on the outer peripheralsurface of the honeycomb segment joined body 10 to function as aprotector for the outer peripheral surface of the honeycomb segmentjoined body 10. Thickness of the outer peripheral coat layer 4 issuitably selected, for example, within the range from 0.1 to 1.5 mm.

A granular filler contained in the bonding material layer 9 and theouter peripheral coating layer 4 may be made of inorganic or organicmaterial. Examples of the inorganic material include glass beads and flyash balloon. Examples of the organic material include starch and foamingresin.

The granular filler preferably has an average diameter of 10 to 300 μm,more preferably 15 to 250 μm, and particularly preferably 20 to 200 μm.In the granular filler, the ratio of the length of the longer centralaxis to that of the shorter central axis is preferably 1.0 to 4.0, andmore preferably the granules are right spherical. The granular filler iscontained in the bonding material layer 9 and the outer peripheralcoating layer 4 at the rate of preferably 20 to 70% by volume, morepreferably 25 to 65% by volume, and particularly preferably 30 to 60% byvolume. The granular filler preferably has a hollow structure. By usinggranules having a hollow structure (hollow granules, density in a jointportion and the outer peripheral surface formed by curing the bondingmaterial layer 9 and the outer peripheral coating layer 4 is lowered,and the Young's modulus can be lowered. This improves thermal shockresistance of the joint portion and the outer peripheral surface, andcracks are inhibited from being generated upon use.

The bonding material layer 9 and the outer peripheral coating layer 4may further contain at least one kind selected from the group consistingof inorganic particles, oxide fibers, and colloidal oxides at the rateof 5 to 60% by mass in addition to the above ceramic and granularfiller. By containing them, the bonding material layer 9 and the outerperipheral coating layer 4 can have improved properties. Examples of theinorganic particles include at least one ceramic selected from the groupconsisting of silicon carbide, silicon nitride, cordierite, alumina,mullite, zirconia, zirconium phosphate, aluminum titanate, and titania;Fe—Cr—Al based metals; nickel based metals; metal Si; and SiC. Examplesof the oxide fibers include aluminosilicate fiber, and other fiber.Examples of the colloidal oxides include silica sol and alumina sol.

The bonding material layer 9 and the outer peripheral coating layer 4preferably has a thermal conductivity of 0.1 to 5.0 W/m·k. It ispreferable that a thermal expansion coefficient of the bonding materiallayer 9 and the outer peripheral coating layer 4 is relatively low inorder to inhibit cracks from being generated due to thermal shock, orthe like, and the preferable range is from 1×10⁻⁶ to 8×10⁻⁶/° C.

EXAMPLE

The present invention will hereinbelow be described more concretely withreferring to Examples. However, the present invention is by no meanslimited to these Examples.

Example 1 Production of Honeycomb Structure

There were produced honeycomb segments each having dimensions of 3.5mm×3.5 mm×150 mmL, a partition wall thickness of about 300 μm (12 mil),and a cell density of about 47 cells/cm² (300 cells/in²).

Production of Honeycomb Structure

A slurried bonding material was applied on the honeycomb segments tojoin the honeycomb segments together and dried and cured to produce ahoneycomb segment joined body, which was then ground. Then, a slurriedouter peripheral coat material was applied to form an outer peripheralcoat layer. Thus, a columnar honeycomb structure for a DPF having adiameter of 144 mm and a length of 152 mm was produced. In this case,the maximum thickness (t₁) of the outer peripheral coating layer wastwice the average thickness (t₂) in the portions other than the portionhaving the maximum thickness.

Examples 2 to 10 and Comparative Example 1

Nine kinds of honeycomb structures were produced in the same manner asin Example except that thickness of the outer peripheral coat layers waschanged as shown in Table 1.

Evaluation

The honeycomb structures obtained in Examples and Comparative Exampleswere put in furnaces heated at 7 kinds of set temperatures shown inTable 7 and kept for one hour until the interior of each of thehoneycomb structure had the set temperature. Then, the honeycombstructures were taken out and observed to confirm presence/absence of acrack generated due to the difference in thickness depending on portionsof the outer peripheral coat layer. In the crack evaluation, “good” wasgiven to the case of no crack, and “bad” was given to the case of havinga crack. The results are shown in Table 1. In Comparative Example 1 (thecase of t₁=t₂ and t₃=t₄), a crack was generated when the set temperaturewas 300° C. In contrast, as in Examples 1 to 9, as thickness of t₁ andt₃ was increased to twice t₂ and t₄ or more (2 to 40 times),respectively, the set temperature where a crack was generated was high.That is, it was understood that the thicker the predetermined portionsof the outer peripheral coating layer was, the more a crack generationlimit was improved.

TABLE 1 Comp. Ex. 1 Example 1 Example 2 Example 3 Example 4 Example 5Example 6 Example 7 Example 8 Example 9 Set temperature t₁ = t₂ t₁ = 2t₂t₁ = 5t₂ t₁ = 10t₂ t₁ = 15t₂ t₁ = 20t₂ t₁ = 25t₂ t₁ = 30t₂ t₁ = 35t₂ t₁= 40t₂ (° C.) t₃ = t₄ t₃ = 2t₄ t₃ = 5t₄ t₃ = 10t₄ t₃ = 15t₄ t₃ = 20t₄ t₃= 25t₄ t₃ = 30t₄ t₃ = 35t₄ t₃ = 40t₄ 200 Good Good Good Good Good GoodGood Good Good Good 250 Good Good Good Good Good Good Good Good GoodGood 300 Bad Good Good Good Good Good Good Good Good Good 350 Bad BadBad Bad Good Good Good Good Good Good 400 Bad Bad Bad Bad Bad Bad GoodGood Good Good 450 Bad Bad Bad Bad Bad Bad Bad Good Good Good 500 BadBad Bad Bad Bad Bad Bad Bad Good Good 550 Bad Bad Bad Bad Bad Bad BadBad Bad Bad

INDUSTRIAL APPLICABILITY

A honeycomb structure of the present invention is useful as a dieselparticulate filter (DPF) for trapping and removing particulate mattercontained in Exhaust gas from, for example, a diesel engine as atrapping filter for exhaust gas.

1. A honeycomb structure comprising: a honeycomb segment joined bodycomprising a plurality of honeycomb segments unitarily joined at jointfaces thereof by means of a bonding material layer, and an outerperipheral coat layer covering an outer peripheral surface of saidhoneycomb segment joined body, the honeycomb structure having aplurality of cells functioning as fluid passages disposed in parallelwith one another in a central axis direction; wherein a thickness of oneportion of said outer peripheral coat layer located on end faces inlongitudinal direction of the bonding material layer in a cross-sectionin a direction perpendicular to the central axis direction is largerthan an average thickness of a first remainder of said outer peripheralcoat layer; wherein the one portion is located on the end faces of saidouter peripheral coat layer; and wherein a thickness (t₃) of said outerperipheral coat layer at the four portions (portions on end faces in 90°directions) located in 90° direction with one another on end faces inlongitudinal direction of the bonding material layer in a cross-sectionin a direction perpendicular to the central axis direction and theaverage thickness (t₄) in a third remainder of said outer peripheralcoat layer satisfy the relation of (t₃)≧(2 to 40)×(t₄).
 2. A honeycombstructure according to claim 1, wherein a cross-section, in a directionperpendicular to the central axis direction, of the honeycomb segmentshas a rectangular shape.
 3. A honeycomb structure according to claim 1,wherein a cross-section, in a direction perpendicular to the centralaxis direction, of the honeycomb segment joined body has a shape of acircle, an ellipse, or a racetrack.
 4. A honeycomb structure accordingto claim 1, wherein the honeycomb segments constituting the outermostperiphery of the honeycomb structure do not have a crust having a pitchof 7 cells or more.